ins_aeas@jhunix.UUCP (Earle A. Sugar) (05/05/87)
Does anybody have the details of how MTS stereo is multiplexed and impressed on the VHF carrier? What I am interested in in building my own decoder to connect to my stereo and TV. Can I just put a normal FM stereo decoder chip in the normal audio signal from the TV tuner , and feed the output through the phono inputs on my stereo? Or is there more to it (probably)?
dplatt@teknowledge-vaxc.ARPA (Dave Platt) (05/05/87)
Posting-Front-End: GNU Emacs 18.41.3 of Tue Apr 7 1987 on teknowledge-vaxc (berkeley-unix) Recent issues of Radio-Electronics magazine contain plans for, and a technical description of an MTS decoder. Briefly, the MTS system is similar in many respects to the FM stereo-subcarrier system, but there are some substantial differences (the subcarrier frequency offset is different, there's a second subcarrier for the "second audio program", and the subcarrier is companded using a variation of the dbx system). Simply using an FM-tuner decoder chip won't work.
cmcmanis@sun.uucp (Chuck McManis) (05/06/87)
In article <4822@jhunix.UUCP>, ins_aeas@jhunix.UUCP (Earle A. Sugar) writes: > Does anybody have the details of how MTS stereo is multiplexed and > impressed on the VHF carrier? What I am interested in in building my > own decoder to connect to my stereo and TV. Can I just put a normal > FM stereo decoder chip in the normal audio > signal from the TV tuner , and feed the output through the phono inputs > on my stereo? Or is there more to it (probably)? There was a fairly complete 2 part article in Modern Electronics about how MTS Stereo was achieved. Basically it used the standard trick of modulating the channels that FM stereo does R+L and R-L on the carrier so regular TVs get mono and by splitting the R-L channel which is modulated by a carrier to put it out of the TVs 'Audio' range you can derive the separate R and L channels. The biggest difficulty is that it is also companded using a dbx (tm somebody I'm sure) scheme to increase the quality of the high end given the limited bandwidth. If you have a 'stereo ready' TV like me then the manufacturer was nice enough to pull the audio out to a connector in the back that hasn't gone through the standard low pass filter (which removes the other stuff). Also note that the equipment in some Cable TV vendors strips this off anyway (being old equipment that is trying to 'clean up' the audio channel). -- --Chuck McManis uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com These opinions are my own and no one elses. But you knew that, didn't you.
bill@videovax.UUCP (05/07/87)
In article <4822@jhunix.UUCP> ins_aeas@jhunix.UUCP (Earle A. Sugar) writes: > > Does anybody have the details of how MTS stereo is multiplexed and >impressed on the VHF carrier? What I am interested in in building my >own decoder to connect to my stereo and TV. Can I just put a normal >FM stereo decoder chip in the normal audio >signal from the TV tuner , and feed the output through the phono inputs >on my stereo? Or is there more to it (probably)? You're right, it's not that simple. To build a proper decoder requires a special IC made by dbx that is not available to consumers. In short, you're better off buying a add-on decoder like F.R.E.D. BTW, the audio subcarrier is 4.5 MHz above the video carrier and most add-on decoders pick up this sound I.F. right at the video detector before it gets trapped out by the TV. Quite some time ago I posted an article about MTS that might help explain how it works. It's a bit dated, but the technical information is accurate. Subject: Stereo TV ----------------------------------- From: net.ham-radio: In article <1178@mhuxt.UUCP> rma@mhuxt.UUCP (ATKINS) writes: >Does anyone know where I can find out the scheme used for >stereo sound transmission on broadcast TV signals? Looking at >the signal it seems to be some kind of sub-carrier system, but >not (L+R) and (L-R) as in FM radio. For those people unfamiliar with FM stereo broadcasting, it uses a system of subcarriers to encode the stereo information. The mono (L+R) information is broadcast as the normal baseband signal. This is done to remain compatible with mono FM receivers. The difference (L-R) is transmitted as a 38 KHz subcarrier using double sideband, suppressed carrier AM (DSB-SC). A 19 KHz pilot tone is provided to allow stereo receivers to decode the subcarrier (also provides an indication that an incoming broadcast is in stereo). To decode the left- and right-channel information, the subcarriers are combined as follows: (L+R) + (L-R) = 2L (L+R) - (L-R) = 2R Thus, a simple matrix is all that's needed to decode stereo FM. Mono receivers filter out the subcariers, which leaves the mono (L+R) component. Now, on to stereo television. Being an EE in the Television Products Division at Tektronix, I have become familiar with the American stereo TV system, known as MTS or BTSC. MTS does transmit L+R and L-R, but not the same as FM radio. First, the frequencies involved are different. MTS uses 15.734 KHz for its pilot and 31.468 KHz for the L-R instead of the 19 and 38 KHz of FM radio. 15734 Hz is the horizontal scanning rate in US TV's, so it was chosen as the basis for MTS to reduce interference between the horizontal circuits and the stereo decoder. TV people call this frequency H. Thus, the pilot is at H and the L-R is at 2H. The principal difference between TV and FM is that the L-R subcarrier in BTSC is compressed before transmission. This was done to reduce noisy reception in fringe areas. Although designed by dbx, the companding system used for stereo TV is not the simple linear 2:1 system used in their tape noise reduction systems. In TV stereo, the companding is both amplitude and frequency dependent. This makes the expander circuit fairly complex and I'm afraid no one has managed to squeeze it into a single IC (yet). (Actually, I have heard of one company that it trying to develop a digital TV stereo chip, but that's probably still a couple of years away). The present decoders have lots of discrete components and about 8 trimpots, so you can see there is a lot of room for improvement. [Now there is a single IC that does the expanding, but is not available to consumers-- WKM 5/7/87] For the sake of completeness, I should mention that BTSC provides for two other subcarriers, SAP at 5H and PRO at 6.5H. SAP stands for Second Audio Program and is intended for bilingual broadcasts (e.g., SAP can broadcast a dubbed foreign language version of the audio). SAP is FM modulated and dbx compressed. I believe the frequency response is somewhere around 12 KHz. The PRO (PROfessional) channel is for data or low-grade voice transmission and is not intended for the general public. So far, there are several stations using SAP (to transmit in Spanish). There are about 250 TV stations broadcasting stereo now, with more on the way soon. Most of these stations carry NBC, the only commercial network using stereo. ABC and CBS plan to go stereo soon. For additional information, see the following articles: "TV Multichannel Sound - The BTSC System," C. G. Eilers, _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30, pp. 236-240, Aug. 1984 "A Companding System for Multichannel TV Sound," L. B. Taylor, M. F. Davis, W. A. Allen, _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30, pp. 633-640, Nov. 1984 I hope this helped! -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 UUCP: ...{hplabs,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "How can I prove I am not crazy to people who are?"